Variable inductor



Jan. 14, 1969 D. R. LOHRMANN 3,422,383

VARIABLE INDUCTOR Filed Oct. 5, 1966 INVENTOR, DIETER R. 'LOHRMANN.

United States Patent 1 Claim ABSTRACT OF THE DISCLOSURE A variable inductor comprising an electrically conductive ribbon, having aninsulator mounted on one side, is wound on a pair of spools which are mounted for rotation in a common plane. A gear train drive rotates simultaneously the spools for transferring the ribbon from one spool to the other. The difference in rotation of the spools is accounted for by a spring which connects one of the spools to the gear train drive. A first electrical terminal is connected to one end of the ribbon and the second terminal is a pair of rollers which embrace that portion of the ribbon which extends between the spools.

The present invention relates to a variable inductor and more particularly to a coil assembly capable of being selectively wound and unwound to vary the inductance thereof.

In the field of radio it has been the practice to provide an impedance matching network between the antenna and the radio set for the efficient transfer of energy between antenna and radio over a large range of operating frequencies. Such matching networks usually use a variable inductor device which comprises some type of coil the turns of which are variable in number. By varying the number of turns, of course, the inductance, which is a monotonic increasing function of the number of turns, also varies. However, while varying the number of turns, the length of the coil would also vary. Since the inductance is a monotonic decreasing function of the length of a coil and since the length of the coil increases as the number of turns are increased, devices capable of relatively large changes in inductance have also been large to permit the length of the coil to increase.

Therefore, those concerned with the development of variable inductors have long recognized the need for a compact, inexpensive variable inductor which can be designed to produce large changes in inductance. The present invention fills this need.

The exact nature of this invention as well as other advantages thereof will be readily apparent from consider ation of the following specification relating to the annexed drawing which shows a perspective view of a preferred embodiment of the invention.

Referring now to the drawing, there is shown a variable inductor having a coil assembly 12 and a supply assembly 11.

Coil assembly 12 comprises a spool 13 mounted in a cylindrical chamber 14 having an opening 15 therein. Chamber 14 is made of a magnetically permative material such as ferrite. A shaft 16 passes through chamber 14 and is joined to spool 13. At one end of shaft 16 are mounted a tuning knob 17 and a gear 18. Shaft 16 is made of a dielectric material such as Teflon. A conductor 19 passes through the center of shaft and communicates with the inner portion of spool 13 where it is connected to a copper strip 20 which is deposited on a layer of dielectric material 21' to form. a ribbon 22 which is wound on spool 13-. A brush 23 contacts conductor 19 to provide a first output terminal 24.

Supply assembly 11 comprises a supply spool 25 mounted freely on shaft 26. Shaft 26 is joined to gear a 27. A coil spring 28 joins shaft 26 to spool 25. Ribbon 22 is wound on spool 25 and spring 28 is normally under tension whereby ribbon 22is maintained under tension.

An idler gear 30 meshes with both 1 gears 18 and 2 1. A pair of rollers 31 and 32 squeeze ribbon 22. Roller 32 is made of conductive material and is in contact with the copper strip 20 on ribbon'y22 to provide a second output terminal 34 which is connected to brush 33 in contact with shaft 32. The portion of the coil formed by conductive strip 20 which extends between the output terminals 34 and 24 constitutes the inductor. The number of turns may be varied by simply rotating knob 17. Both spools 13 and 25 will rotate and spring 2 8Lwill maintain the ribbon 22 under tension. The chamber 14 increases efficiency by preventing magnetic energy, produced by coil assembly 12, from. creating eddy currents in the surrounding equipment such as the radio housing, etc.

At the higher frequencies the capacitance between successive turns of the coil may become" a problem. The ribbon may be designed to reduce the capacitance by in creasing the thickness of the dielectric 21 and/ or substituting a material with a lower dielectric constant. For example, short spacer elements may be spaced along copper strip 20 (like railroad ties) such that the majority of the dielectric between the turns in the coil is air.

Obviously many modifications and gvariations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the the scope of the appended claim,the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A variable inductor comprising; a first spool having a first axial shaft rigidly connected thereto; a second spool mounted for rotation about a second axial shaft; a spring means connected between said second spool and said second shaft for providing a rotational bias between said second spool and said second shaft; a supporting structure; said first and second shafts being rotatably mounted on said structure, with said spools lying in a common plane; a relatively thin conductive ribbon having an insulating means thereon; said ribbon extending under tension between said spools and being wound about said spools, to form coils in a single plane drive; drive means extending between said shafts for simultaneously rotating said shafts; a first electrical contact means extending from one end of said conductive ribbon; a second electrical contact means including a pair of rotatable rollers mounted on said support between said spools; said ribbon which extends between said spools passing between and being squeezed by said rollers; and a magnetically permative container -=substantia1ly enclosing the c011 extending be FOREIGN PATENTS tween 831d tepmmals- 588,413 12/1959 Canada.

References Cited 1,271,815 8/1961 France.

UNITED STATES PATENTS 5 LEWIS H. MYERS, Primary Examiner. 2,659,057 11/1953 Legg et a1 33615 T. J. KOZMA, Assistant Examiner. 2,659,794 11/1953 Mairs 33615 XiR 2,919,400 12/1959 Hartung 33879 US. Cl. X.R. 2,982,929 5/ 1961 StOWer 33 6-I5 338-79 2,988,638 6/1961 Knausenbenger 334--38 10 

